Amplitude control circuit for electrical instruments



July 7, 1970 J. c. COOKERLY ETAL 3,519,724

AMPLITUDE CONTROL CIRCUIT FOR ELECTRICAL INSTRUMENTS I Filed Dec. 8. 1967 A v INSTRUMENT K' L II I I B WAVE SHAPE II A L c m" TRANSDUCER wAvE SHAPE I I I AMPLITUDE IT I T2 26 25 K A I A TIME I I a? I I B j TIME I C 28 I 32 I 29 TIME I if i ziftt." 30 l 1 '3I' D TIME FIG 2 II'VVFN'I'UII.

JACK C. COOKERLY Y GEORGE R. HALL WEW ATTORNEYS United States Patent 3,519,724 AMPLITUDE CONTROL CIRCUIT FOR ELECTRICAL INSTRUMENTS Jack C. Cookerly, 7655 Atoll Ave., North Hollywood,

Calif. 91605, and George R. Hall, 13613 Huston,

Sherman Oaks, Calif. 91403 Filed Dec. 8, 1967, Ser. No. 689,153 Int. Cl. Gh 1/02 US. Cl. 841.27 10 Claims ABSTRACT OF THE DISCLOSURE In an electrical musical instrument in which an original tone is transduced into an A.C. signal and the A.C. signal in turn passed through a wave shaping circuit to provide a modified signal, an amplitude control circuit is provided to vary the amplitude of the modified signal in accord with amplitude changes of the original tone. The modified signal is ordinarily derived from a wave shaping circuit of the type which is triggered such that its direct output is of a constant amplitude regardless of variations of the exciting A.C. signal. The amplitude control circuit in accord with the invention includes a diode functioning as a variable impedance connected to receive the constant amplitude modified signal and provide an output-signal to a suitable sound reproducing system. The amplitude of the output signal is determined by the effective im" pedance of the diode. This impedance in turn is determined by a control signal connected to control a reverse bias to the diode, the control signal varying in amplitude in accordance with amplitude variations in the A.C. signal applied to the wave shaping circuit. A transistor circuit is provided with the base of the transistor receiving a rectified form of the A.C. signal and the control signal being derived from the collector terminal of the transistor. The overall result is a control of the ultimately reproduced sound in loudness which corresponds to the loudness of the original tone. The circuit includes means for varying the threshold level at which it operates and also means for varying the dynamic range of the output signal.

This invention relates generally to electrical musical instruments and more particularly to an improved circuit for controlling the amplitude of one or more modified signals derived from one or more wave shaping circuits in the musical instrument in accord with amplitude variations of an original tone played by a musician.

Musical instruments of the type under consideration include one or more wave shaping circuits to provide modified electrical signals derived from an original A.C. signal representing an original tone played by a musician or otherwise generated in the instrument. The wave shaping circuit or circuits function to provide unique effects in the reproduced sound from the instrument. For eX- ample, an original tone may be transduced into an A.C. signal such as a sine Wave. This sine wave in turn may be passed through a Wave shaping circuit to provide a square wave output which, when reproduced, will have a quality different from that which would normally be produced by the sine wave. Thus various different instruments may be simulated by a single electrical musical instrument by use of the wave shaping circuits.

A more complete description of the foregoing types of instruments is set forth in our US. Pat. No. 3,213,180, issued Oct. 19, 1965.

Usually the wave shaping circuits are only responsive to the frequency of the input signal and are independent of the amplitude of the input signal. The modified output signal derived from the wave shaping circuit is thus of a constant amplitude although the frequency may have 3,519,724 Patented July 7, 1970 ice been doubled or halved and the particular shaping of the wave form modified to thus vary the quality of the ultimately reproduced sound. In other instances, the instrument may incorporate a keyboard wherein electrical signals generated by oscillators are connected to a sound reproducing means in response to a musician closing one of the keys of the keyboard. Since the key merely functions to close a circuit, the degree of pressure or attack applied by the musician on the keyboard will not affect the amplitude or loudness of the ultimately reproduced sound. In these instances, there may be provided other means for injecting a signal which varies as a function of the musicians attack or desired amplitude control for the particular tones to be played.

The present invention has as its primary object the provision of an improved amplitude control circuit for an electrical instrument wherein the amplitude or loudness of ultimately reproduced sounds derived from constant amplitude modified signals whether constituting the outputs of wave shaping circuits or direct outputs from tone generating oscillators, is varied in accord with original amplitude variations of the original tone.

More particularly, it is an object to provide an amplitude control circuit meeting the foregoing object wherein a desired threshold adjustment is provided for defining a threshold level of amplitude below which no output signal is passed to the sound reproducing means.

Another object is to provide an amplitude control circuit means for varying the dynamic range of the ultimately reproduced sound for a given dynamic range characterizing the originally played tone.

Other objects of this invention are to provide an amplitude control circuit for electrical instruments meeting all of the foregoing objects in the form of solid state circuitry adaptable to the controlling of one or several modified output signals from one or several wave shaping circuits, which employs relatively few elements and yet is designed for temperature compensation, and which is compact and relatively economical to manufacture.

Briefly, these and other objects and advantages of this invention are attained by providing a variable impedance means connected to receive the constant amplitude modified signal such as from a wave shaping circuit in an electrical musical instrument, and provide an output signal having an amplitude which is a function of the effective impedance of the variable impedance means. A converting control means in turn provides a control signal to the variable impedance means, the effective impedance varying as a function of the amplitude of this control signal. The amplitude of the control signal in turn is caused to vary in accordance with amplitude variations in an A.C. signal received in the converting and control means. This A.C. signal is provided from a transducer and varies in amplitude in accordance with the amplitude variations in the original tone.

The overall result is a desired variation in the amplitude or loudness of the ultimately reproduced sound corresponding to variations in the amplitude of the original tone as intended by the musician. The converting and control means, in its preferred embodiment, also includes threshold level adjustment means and dynamic range control means so that considerable versatility is provided with respect to ultimate control of the reproduced sounds.

The foregoing as well as further features and advantages inherent in the circuit of the present invention will become clearer by now referring to a preferred embodiment thereof as illustrated in the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram partly in block form illustrating the invention; and

FIG. 2 illustrates a series of wave forms useful in explaining the operation of the circuit of FIG. 1.

Referring first to FIG. 1 there is illustrated by the block a musical instrument which may constitute an electric guitar, organ, wind instrument, or even simply a micro phone. Associated with the instrument 10 is a transducer 11 for converting original tones provided by the instrument 10 or by a musician into an A0. signal. This A.C. signal will have a frequency and amplitude constituting a function of the pitch and loudness or volume of the original tone from the instrument 10.

As shown in FIG. 1, the AC. signal from the transducer 11 passes to a distribution line 12 and thence to one or more wave shaping circuits such as indicated at 13, 13, 13". These Wave shaping circuits may be of the type described in our aforementioned US. patent wherein the outputs constitute modified signals of constant amplitude; for example, a square wave as from the output of the wave shaping circuit 13.

As shown in FIG. 1, the output from the wave shaping circuit passes to a variable impedance means encompassed generally within the dashed line 14. From the variable impedance means 14, an output signal derived from the constant amplitude modified signal passes to a sound reproducing means which may constitute a simple audio amplifier 15 and loudspeaker 16. The outputs from any further additional number of wave shaping circuits such as 13 and 13 similarly pass to variable impedance means indicated by the dashed boxes 14', 14", and so forth.

As will become clearer as the description proceeds, the output from the variable impedance means 14 has an amplitude which is a function of the effective impedance of the circuit 14. Thus by changing this impedance in accord with amplitude variations of the original tone, the output signal will vary in amplitude in accord with the original tone.

The foregoing control of the impedance of the vari-- able impedance means is effected in accord with the circuit of FIG. 1 by a converting control means. This latter means includes a branch lead 17 for passing the A.C. signal from the transducer 11 of FIG. 1 through a DC. blocking condenser C1 to a junction point 18 constituting the anode of a first diode D1. The cathode of this diode connects to the base terminal 19 of a transistor Q1. The collector and emitter terminals for the transistor Q1 are indicated at 20 and 21 and connect respectively through first and second resistances R1 and R2 to a D.C. power supply PS and ground 22.

A threshold adjustment means is provided in the converting control means for establishing a voltage level at the base of the transistor 31 below which the transistor Q1 is non-conducting. This threshold adjustment means includes resistances R3 and R4 in series with a second diode D2 and a resistance R5. These elements define a voltage dividing circuit connected between the power supply PS and ground as shown. A given threshold level of voltage is tapped from the junction point between the resistance R4 and the second diode D2 and connected to the anode point 18 of the first diode D1 as shown. The AC. signal from the line 17 also connected to the anode 18 of the diode D1 is superimposed on the threshold voltage level established at this point by the threshold adjustment means.

When the transistor Q1 is conducting, there is established at its collector terminal 20 a control signal. This control signal will vary in amplitude in accord with variations of the input voltage on the base of the transistor Q1. The rate of variation of the control signal on the collector 20 may be adjusted by the emitter resistance R2 which is indicated as variable. As will also become clearer as the description proceeds, this variable resistance in the emitter circuit of the transistor enables an adjustment of the dynamic range of the volume control of the ultimately reproduced sound. Condensers C2 and C3 connecting from the base terminal 19 is ground and the collector terminal 20 respectively function to provide reverse feedback for the transistor and remove tny A.C. signal from the control signal at the collector 20.

The referred to control signal from the collector 20 is passed through line 23 to the variable impedance means 14. This variable impedance means includes a third diode D3 defining a series circuit with a resistance R8 connected to its anode and a capacitor C5 connected to its cathode. The output from the wave shaping circuit 13 passes through the resistance R8 to the anode of the diode D3 and applies a positive forward bias V+, less than the power supply voltage, to D3. The output signal from the diode and capacitor C5 passes to the sound reproducing means 15 and 16. Essentially, the output signal has an am plitude determined by the effective impedance of the diode D3. Therefore, by effectively varying the impedance of the circuit the amplitude of the output signal from the capacitor C5 is varied.

The foregoing control of the amplitude of the output signal by means of controlling the eifective impedance of the diode D3 is accomplished by the control signal on the lead 23. This control signal is passed through a coupling network comprising resistances R6 and R7 and a condenser C4 to ground to the cathode 24 of the third diode D3 and thus applies a reverse bias on the diode D3. The effective impedance of the circuit is controlled by the amount of reverse bias applied and thus the amplitude variations of the output signal from the variable impedance means can be controlled by amplitude variations in the control signal at the cathode 24 of the diode.

A better understanding of the foregoing as well as the overall operation of the circuit of FIG. 1 will be had by now referring to the various wave forms illustrated in FIG. 2. These Wave forms are lettered respectively A, B, C, and D and represent the wave forms occurring at the correspondingly lettered points in FIG. 1.

Referring first to the wave form A there is illustrated at 25 the AC. signal derived from the transducer 11 of FIG. 1 and occurring at the point A of FIG. 1. For purposes of illustrating the operation of the invention, it is assumed that the loudness of the original tone received by the transducer is increased between the times T1 and T2 in the wave forms of FIG. 2 so that the amplitude of the resulting A.C. signal increases as indicated at 26.

Referring to the next wave form B, there is illustrated at 27 the square wave output from the wave shaping circuit 13. It will be noted that the modified output signal is of constant amplitude notwithstanding the change in amplitude of the input exciting A.C. signal.

Wave form C represents the control signal appearing on the lead 23 from the collector 20 of the transistor Q1. It will be noted that this control signal has a DC. level 28 for an input A.C. sine wave signal as shown at 25. As the amplitude of the AC. signal increases as between the times T1 and T2, the rectified form of this signal appearing on the base terminal 19 of the transistor Q1 will increase thereby increasing the conduction of the transistor Q1. With increased conduction of current through the transiistor Q1, the potential at the collector terminal 20 will drop as a consequence of the increased voltage drop across the power supply resistance R1. Thus the voltage at the collector terminal will decrease as indicated at 32 between the times T1 and T2 in FIG. 2. When the new amplitude 26 is attained by the AC. signal, the control signal at the collector will be as indicated at 29.

Wave form D illustrates the output signal from the variable impedance means 14 passed to the amplifier 15 and speaker 16. As shown in FIG. 2, this output signal will have an amplitude 30 corresponding substantially to the amplitude of the original A.C. signal portion 25. In this respect, the reverse bias applied to the cathode 24 of the third diode D3 in the variable impedance means 14 is defined by the voltage level 28 of wave form C and for this particular value of reverse bias, the amplitude of the output signal is as illustrated in wave form D. When the reverse bias is decreased as a consequence of the volume change in the A.C. input signal and as described at 32 in FIG. 2, the output signal from the diode D3 as a consequence of the effective change in the impedance of the diode will increase as indicated at 31 in FIG. 2. It will thus be apparent that the amplitude variations occurring in the original A.C. signal are restored to the output signal passed to the sound reproducing means.

As mentioned heretofore, the threshold level at which the transistor Q1 will commence conducting can be controlled. This adjustment can be effected by the variable resistance R4 in FIG. 1 and is desirable in order to cut off the output signal when the A.C. input signal drops below a given amplitude level. For example, by establishing .6 volt at the anode 18 of the first diode D1, the threshold of conduction of the transistor Q1 may be established at a given voltage difference from .6 volt. This voltage difference must be made up by the amplitude of the rectified sine wave passed through the diode D1 which signal is superimposed upon the threshold level of .6 volt. Conduction of the transistor Q1 cannot take place accordingly unless the amplitude of the A.C. signal exceeds a given level and therefore a desired threshold level is established. Similarly, when the amplitude of the A.C. signal drops below a given level, conduction of the transistor Q1 will be cut off thus blocking the output signal as a consequence of the high reverse bias applied thereto by the control signal 23 this reverse bias amounting to substantially the DC. voltage value of the power supply when the transistor Q1 is not conducting.

Referring once again to FIG. 2, the manner in which the dynamic range of the output signal can be controlled will be described. In FIG. 2 there is illustrated by the dashed lines for the wave form C a change in the control signal as indicated at 32' to a final amplitude level as indicated at 29'. This change in the control signal is effected by adjustment of the variable resistance R2 in the emitter circuit of the transistor Q1. Thus, changing this resistance will vary the rate of change or slope of the line 32. As a consequence, the rate of change of the amplitude of the output signal with a change in the amplitude of the A.C. signal can be varied as indicated by the increased amplitude of the output signal shown in dotted lines at 31'. Thus, by varying the emitter resistance R2, the dynamic range of the ultimately reproduced sound can be varied as desired.

In addition to the foregoing, the circuit of FIG. 1 provides further unique advantages. For example, the thermal characteristics of the transistor Q1 and diode D2 are made such that temperature compensation is achieved. In other words, any variation in temperature which might change the threshold of conduction of the transistor Q1 also affects the threshold level established by the second diode D2 at the point 18 to compensate for such change. Thus the circuit is stable from a temperature variation standpoint within contemplated limits.

The converting control means for establishing the control signal passed to the variable impedance means 14 is such that the various other variable impedance means 14 and 14" illustrated in FIG. 1 may be paralleled to the output from the collector as indicated by the dashed line L connecting to the lead 23. A single converting control circuit can thus be employed for controlling the ultimately reproduced sound volume from a multiplicity of wave shaping circuits which might be incorporated in the instrument.

Finally, the particular circuit configuration as described employs only a single transistor and three diodes as the active control elements and thus the entire circuit is inexpensive to manufacture and compact in configuration.

While the invention has been described with respect to the block of FIG. 1 representing a musical instrument, it should be understood that such instrument could be a human singing voice and the transducer 11 simply a microphone. Moreover, equivalent control of the amplitude can be achieved by employing a field effect transistor in place of D3 to provide essentially a variable resistance of impedance.

From the foregoing description, it will be evident that the present invention has provided a unique amplitude control circuit for electrical instruments wherein all of the objects heretofore set forth are fully realized.

What is claimed is:

1. In a musical instrument including transducer means responsive to playing of said instrument for providing an A.C. signal the frequency of which is a function of the pitch of an original tone resulting from the playing of said instrument and the amplitude of which is a function of the loudness of said tone as controlled by the player of said instrument, and including a wave shaping means receiving said A.C. signal and providing a constant amplitude modified signal, an amplitude control means for varying the amplitude of said modified signal in accordance with amplitude variations of said A.C. signal so that ultimate reproduction of sound from said modified signal will vary in loudness in accordance with said original tone, comprising, in combination: variable impedance means connected to receive said constant amplitude modified signal and provide an output signal derived from said constant amplitude modified signal but having an amplitude which is a function of the effective impedance of said variable impedance means; a sound reproducing means connected to said variable impedance means to receive said output signal; and a converting control means connected to said variable impedance means and responsive to said A.C. signal to provide a control signal which varies as a function of the amplitude of said A.C. signal, said converting control means including a transistor having base, collector, and emitter terminals; means connected to said base terminal receiving said A.C. signal, said control signal appearing at the output of said transistor and effectively varying the impedance of said variable impedance means to thereby vary the amplitude of said output signal, whereby ultimate reproduction of said output signal by said sound reproducing means varies in loudness in accordance with said original tone.

2. The subject matter of claim 1, in which said converting control means includes threshold adjustment means for providing a threshold level of amplitude below which no output signal is passed to said sound reproducing means.

3. The subject matter of claim 1, in which said converting control means includes dynamic range adjustment means for varying the rate of change of said control signal with change in the amplitude of said A.C. signal whereby the dynamic range of the sound reproduced by said reproducing means may be adjusted for a given dynamic range of said original tone.

4. The subject matter of claim 1, including means for connecting said converting control means to additional variable impedance means which receive, respectively, constant amplitude modified signals from additional Wave shaping means derived from said A.C. signal, such that multiple output signals may be reproduced as sound with variable loudness in accordance with said original tone.

5. The subject matter of claim 1, in which said means connected to said base terminal comprises a first diode receiving said A.C. signal and passing a rectified form of said signal to said base terminal, said control signal appearing at the collector terminal of said transistor.

6. The subject matter of claim 5, in which said converting and control means includes threshold adjustment means in the form of a resistance and a series connected second diode defining a voltage dividing circuit connected between said .power supply and ground, and a lead connecting the junction point between said resistance and second diode to the input of said first diode receiving said A.C. signal, the voltage applied to said first diode being such as to hold said transistor base at a desired threshold level below which said transistor is non-conducting, any A.C. signal received through said first diode being superimposed on said threshold level.

7. The subject matter of claim 5, in which said emitter resistor is variable whereby a dynamic range adjustment means is provided by varying said emitter resistor to vary the rate of change of said control signal on said collector with change in the amplitude of said A.C. signal.

8. The subject matter of claim 5, including condenser means coupling said collector terminal with said base terminal and said base terminal to ground to provide inverse feedback and function to remove any A.C. signal from said base terminal.

9'. The subject matter of claim 6, in which the thermal characteristics of said transistor and said second diode are such that variations in said threshold level and transistor threshold of conduction with temperature are compensated.

10. The subject matter of claim 6, in which said variable impedance means includes a resistance, a third diode, and a capacitor connected in series between said wave shaping means and said sound reproducing means; and a coupling network connected to pass said control signal to the juction point of said third diode and capacitor such that said control signal applies a reverse bias on said diode opposing the forward bias applied by said constant amplitude modified signal, variation in the amplitude of said reverse bias causing an inverse variation in the output signal from said third diode passed to said sound reproducing means such that said third diode functions in effect as a variable impedance whose value is a function of said reverse bias.

References Cited UNITED STATES PATENTS 3,256,381 6/1966 Cookerly et al. 84-l.27

HERMAN KARL SAALBAC-H, Primary Examiner S. CHATMON, JR., Assistant Examiner U :8. Cl. X.R. 

